Lernaean Hydra

It was a gigantic, nine-headed water-serpent, which haunted the swamps of Lerna. Heracles was sent to destroy her as one of his Twelve Labours, but for each of her heads that he decapitated, two more sprang forth. So with the help of Iolaus, he applied burning brands to the severed stumps, cauterizing the wounds and preventing the regeneration. In the battle he also crushed a giant crab beneath his heel which had come to assist the Hydra. 

Parents: Echidna and Typhon
Killed by: Heracles

Hercules and the Lernaean Hydra. Detail of The Twelve Labours Roman mosaic from Llíria, Valencia.